Gain, gain, and more gain.... combined with how they achieve it. IMO... each series added more and more gain. The JMP MV Marshall is "more or less" a Super Lead with a Master Volume... but by the time the JMP MV MKII came out they were cascading gain stages on both the 50 and 100 watt models. This ultimately became the flagship of the JCM800 line. The JCM900 added more gain than the 800, and it kept going from there. A JCM900 does not sound like a JCM800 to me... just as the JCM800 does not sound like a Super Lead, commonly referred to as a PLEXI. I know it's unfair... but I lose interest in Marshalls beyond the JCM800 era.